Devil Arm

Devil Arms are demonic, living weapons that are commonly the manifestation of a defeated Devil's soul or power. In the Devil May Cry series, Devil Arms are considered to be the characters' most powerful weapons. When wielded, Devil Arms often convey their power to the wielder, provided one can subdue them first.

Prelude
The exact process by which a Devil Arm is created is unknown. The first known Devil Arm, the sword Sparda, was created by the Dark Knight Sparda. It is a demonic weapon capable of transforming itself to various types of weapons. It, and other Devil Arms like it, is capable of both opening and sealing Hell Gates, allowing a path between the Human World and Demon World to be opened or shut.

The Dark Knight Sparda used a different Devil Arm, the Yamato, to seal the demon world away from the human world. He also split the power of the sword Sparda by creating the Perfect Amulet from it, causing his power and the sword to enter into a dormant state. Although the exact reasons for this are unknown, it has been speculated that it is harder for more powerful demons to exist in the human world without the presence of a Hell Gate. Thus, by sealing his power, Sparda made it easier for himself to live in the human world. He had two sons by his human wife, Eva, and gave Yamato to Vergil, the first born, and another of his Devil Arms, Rebellion, to Dante, the younger twin.

Devil May Cry 3
Dante now wields Rebellion as he works against his brother. Vergil attempts to re-open the Hell Gate sealed by their father in his quest for ultimate power. Vergil succeeds in raising the Temen-ni-gru, but Dante is able to stop him before he can destroy the human world. Both Dante and Vergil acquire new Devil Arms by defeating the demons guarding the Temen-ni-gru. Each Devil Arm not only grants the twins new powers and abilities, but also seems to exert some influence over their Devil Triggers, as their physical shape in their Devil Trigger form changes depending on the Devil Arm they wield.

Devil May Cry
The Force Edge is used by Dante as his default weapon in the first game. An alternative Devil Arm, the Alastor, can also be found, with the capability of giving Dante new demonic powers. In order to wield the Alastor, Dante had to first prove himself to the spirit of the blade. He does this by surviving being impaled through the chest by Alastor. Yet another Devil Arm, the Ifrit, is also discovered. It lifts Dante off the ground by his arms and tries to pull him in two directions at once, but Dante is too strong for it to succeed, and it submits to him. Dante also recovers both halves of the Perfect Amulet and re-awakens the Force Edge, turning it back into Sparda.

While on Mallet Island, the Devil Arms Dante uncovers also alter his Devil Trigger form, just like the ones from the Temen-ni-gru.

Devil May Cry 4
In Devil May Cry 4, Devil Arms are used by The Order of the Sword to "fuel" several Hell Gates and keep them open. Only a few strong demons are able to use these Hell Gates to enter the human world, and when they are defeated, they do not relinquish their souls as Devil Arms. Instead, Dante takes the Devil Arm holding the gates open.

For the first time, Dante's Devil Trigger form does not change depending on the Devil Arm he is wielding. It is possible that, by this time, Dante has become so powerful that his Devil Arms can no longer influence his demonic form.

Devil May Cry 2
Dante only uses one Devil Arm on his journey to Dumary Island, his trusty Rebellion. He also displays a new Devil Trigger form. Whether or not this form is only available to him with Rebellion as his Devil Arm is unknown.

DmC
In the new game developed by Ninja Theory, the new Dante has only one magical weapon, also called Rebellion capable of transforming to various weapons. Thus, it functions much like the awakened Sparda.

Trailers also show a large demon wielding a weapon which resembles a "grappling claw" which Dante latter wields in-game, alluding to a possible return of Devil Arms in the new DmC.